By this point, I think everybody has their own opinion about what spikes actually do. So, here's an independent article from on the web:

http://www.st-and.demon.co.uk/AudioMisc/cones/speak.html

I, for one, agree with most of the points in the article. I still think that if you're looking to reduce muddiness you're better off isolating the speaker and/or stand from the floor by using something like the Auralex GRAMMA mentioned above.